1. Field of the Invention
The present invention relates to an automatic calibration system for a signal generator incorporating a voltage controlled oscillator. Specifically, the invention relates to a calibration system for recalibrating a look-up table stored in a programmable memory which look-up table stores values representing particular frequency steps from the voltage controlled oscillator.
2. Description of the Prior Art
In general, prior art signal generators use a voltage controlled oscillator to provide for a desired output frequency. One commonly used system includes a phase lock loop design incorporating a reference frequency as a first input to a phase detector. The output from the voltage controlled oscillator is applied through a divide network as a second input to the phase detector. The error signal output from the phase detector is applied to a loop filter and the output from the loop filter is a voltage equivalent of the error signal. The voltage output from the loop filter is used to modify the voltage controlled oscillator to produce the proper output frequency in accordance with the desired output frequency as represented by the value "N" of the divide network. Therefore, when it is desired to change the output frequency to a particular frequency, the divide network is adjusted to a particular value "N" which will in turn control the value of the error signal applied to the voltage controlled oscillator. The frequency of the voltage controlled oscillator will then slew to the particular frequency.
It can be seen that if it is desired to make large changes in the output frequency of the voltage controlled oscillator, it would be quite cumbersome to merely depend upon the single error signal to control the output frequency from the voltage controlled oscillator. In order to more rapidly change to the different frequencies, prior art systems have used stored values which represent stepped frequency values, such as one megahertz steps.
These stored values require a memory, such as a read-only memory with particular stored values. These stored values may be converted to voltage values using a digital-to-analog converter and applied to the voltage controlled oscillator so as to provide the proper control of voltage controlled oscillator. In a sense, the stored values represent a pretuning of the phase lock loop to the expected frequency steps and with a final tuning provided by the error signal produced by the phase detector. In this way the time response of the loop can be greatly increased since the switching to the different frequencies is more rapidly provided when the error voltage from the phase detector is kept as close to a zero value as possible. The prior art systems, therefore, provided for an improvement in the time response of the loop by applying the pretuned values to the voltage controlled oscillator as the divide network is changed and using the phase detector in the loop to make the small corrections to produce the final, more accurate, output frequency.
The closer the stored values representing the stepped frequencies are to the actual values, then the more accurate and quicker the frequency output may be provided for the desired frequencies. Typically, the stored values are customized for each individual frequency generator. The customizing is provided by using custom tables which are stored as a look-up table in a read-only memory such as an EPROM or a PROM. This look-up table is provided as part of a microprocessor and with the stored values provided in digital form and representative of particular voltage values which in turn are representative of particular frequency steps. When it is desired to provide a particular frequency output, the microprocessor accesses the read-only memory look-up table and provides for a digital value representative of this frequency. This digital value is then converted by the digital-to-analog converter into an analog voltage which in turn controls the voltage controlled oscillator to provide for the appropriate frequency output signal. Of course, the final tuning of the frequency signal is accomplished through the phase detector loop to produce a small error signal to make the final adjustment in the frequency.
Periodically, the look up system must be recalibrated because of aging in the system. In this way, the read-only memory such as the EPROM or PROM can be updated to reflect voltage values which are more closely representative of the desired steps after the aging process. In the prior art, this recalibration generally required that the signal generator be sent back to the manufacturer for recalibration. This is because the EPROM cannot be modified under the control of a microprocessor and can only be reprogrammed completely after first being erased by the use of an ultra-violet light source. The PROM cannot be modified at all once it has been programmed and so a new PROM must be programmed and inserted in the signal generator. In general, the manufacturer would check the voltage controlled oscillator and provide for a new read-only memory such as reprogrammed EPROM or a new PROM which now has the proper values to provide for the desired frequency steps. It can be appreciated that this process would be time consuming and expensive and more importantly, the signal generator equipment would be lost to the user for a considerable period of time during recalibration.